The locking unit described before, in particular transmission locking units, are used with vehicles equipped with automatic transmissions or semi-automatic transmissions. The locking units are here supposed to hold the parking brake open during operation. The arrangement is here chosen such that, when the vehicle is ready to run, the vehicle-intern transmission control element system generates hydraulic pressure that acts on an operating element, for example a piston. The piston then pushes by its movement on an armature of the locking unit, and puts it against the pole face of the magnet. The known locking units have a coil with a wire flown through by electric current. The coil is permanently electrified, and the resulting magnetic field develops an appropriate magnetic holding force that attracts the armature with sufficiently high power, and thus defines a first position of the armature rod. The armature rod here interacts with an operating element locking device that holds reliably on to the operating element in this position. This arrangement makes sure that even during lowering or when the hydraulic force acting on the piston is dropped, this does not fall back in its basic position and blocks the vehicle. The known locking unit locks the parking brake in the open condition. Only when the coil current is switched off, the magnetic holding force is reduced so strongly that the locking system opens against the force of a pull-back spring and releases the piston.
The operation of the known locking unit causes a permanent current flow through the windings of the coil. Energy is required.